I have the Belkins $20 cooler which I use everyday but, not so much because of the fan but, more so because anytime I put my MBP Unibody on either my lap or on the bed or any type of fabric cloth material it starts to get warm and gets hotter the longer u leave it there. I could never put my MBP on my lap if it weren't for this cooler even though I rarely use the fan itself. Once I get to the office and place my MBP on a hard surface this becomes a non issue though.
This is actually the second one I've owned. Good value for $20.
I have one. It's a Targus model #AWE41CA. It is alright. I don't use it often. I never noticed a significant difference in temperature (I use iStat menus). But it's good for placing your MBP on if you are gonna have it on your lap. I don't know about everyone else, but mine gets quite hot if i've been using it for a bit.
Most of the time I just use my MBP at a desk. If you like to sit on a couch or something with your MBP, then it's useful to have a cooler. Anything cheap will do; heck, a phonebook would probably work.